The church used several creeds: the Reformed Heidelberg Catechism, Martin Luther’s catechisms, and the early Lutheran Augsburg Confession; Evangelical and Reformed leaders allowed great latitude in interpretation. In the main, Evangelical and Reformed congregations emphasized piety and service rather than legalistic soteriology or orthodox dogma. Styles of worship ranged from revivalism (especially in Ohio and North Carolina) to a Lutheran-like liturgicism (the "Mercersburg Movement," found primarily in central Pennsylvania parishes). Generally speaking, the theological outlook of most ministers was largely accepting of liberal trends in Protestant doctrine and higher Biblical criticism, although some pockets of conservative revivalistic Pietism and confessionalist Calvinism could be found.
Up until the mid-19th century, the Reformed churches ministered to German immigrants with a broadly Calvinist theology and plain liturgy. However, revivals, inspired by Anglo-Saxon Protestant churches during the Great Awakenings of the late 18th and early 19th centuries, influenced the development of the Reformed churches, especially in frontier regions. Some of the more radical practitioners of revivalism and/or pietism defected to Brethren bodies; still others formed the Churches of God, General Conference, a conservative, doctrinally Arminian group.
A backlash set in, however, against revivals in the form of the "Mercersburg Theology" movement. Named for the Pennsylvania town where the Reformed seminary was located in the mid-19th century, scholarly and ministerial advocates of this position sought to reclaim an older, European sense of the church as a holy society that understood itself as organically related to Christ. This implied a recovery of early Protestant liturgies and a renewed emphasis upon the rite of Holy Communion, somewhat akin to the Tractarian or Anglo-Catholic movement in Anglicanism but within a Reformation vein. Some leaders, however, saw this platform as an attempt to impose heretical Catholic practice and understandings in a Protestant setting. This group, centered in southeastern Pennsylvania in close proximity to a large Catholic population in Philadelphia and thus motivated by Anti-Catholicism, objected strenuously to the Mercersburg reforms, going so far as to establish a separate seminary; the school is now known as Ursinus College. After temporarily causing the Ohio Synod to withdraw from the church, tensions mounted until compromises were worked out, and parishes of either low or high persuasion were allowed to practice their preferences peacefully.
A later group, settling in the late 19th century, took root in Wisconsin and spread westward across the Great Plains region; this group spoke German for several generations after the "Pennsylvania Dutch" had thoroughly Americanized themselves, theologically as well as linguistically. These immigrants did not participate in the Mercersburg/Ursinus struggle mentioned above; their theological persuasion was decidedly confessionalist, holding to a fairly strict intrepretation of the Heidelberg Catechism. So strong were the convictions of some that a few churches in that group, most of which were in South Dakota, defected immediately prior to the 1934 merger, influenced by such strict confessionalism, a belief in biblical inerrancy, and a fear of losing their Reformed roots; that group retained the name Reformed Church in the United States for itself.
This schism aside, by the time of the merger talks, the RCUS had mostly joined the American Protestant mainline, sending missionaries overseas and operating health and welfare institutions (i.e., hospitals, orphanges, nursing homes) throughout much of the U.S. Further, the Reformed did some work among Native Americans in Wisconsin. The RCUS' constituency composed slightly over half of the membership of the new denomination in 1934.
Although their faith was chiefly the product of a forced union by the government in Prussia, the Evangelicals by conviction wished to minimize the centuries-old points of contention between Lutheran and Reformed doctrine and practice. This attitude of moderation was enabled in large measure by the rise of pietism, which stressed a more emotional, less rationalistic approach to the teachings of the Bible, thus disinclining scholars and pastors toward technicalities or polemics. Many Evangelical parishes were founded by pastors trained in interdenominational missionary societies such as the one in Basel, Switzerland in the early 19th century; they immigrated to the U.S. to assist settlers fleeing Prussian militarism.
Even to a greater degree than the Reformed, the Evangelicals became most noted among American Protestants for their establishment and staunch support of hospitals, orphanages, and homes for the elderly. Probably most similar in ethos (among English-speaking Protestant groups) to the Methodists, pastors emphasized pietist preaching and catechizing young people for the rite of confirmation, a rite still cherished highly to this day by congregations deriving from ESNA roots. Reflecting a later generation of immigration, the German language persisted for several generations in most congregations before such services were gradually phased out in the era between the World Wars, due in part to anti-German sentiment among some Americans.
In terms of governance, the Evangelicals most resembled American Lutheranism of the time, with high regard for the pastor's authority, but essentially congregational in structure, with a lay council handling temporal matters such as property and benevolences.
President Theodore Roosevelt attended Washington D.C.'s Grace Reformed Church, an E&R congregation. Roosevelt originally belonged to the Reformed Church in America, a Dutch-American group. Since there were no RCA congregations in Washington, he chose Grace Reformed as perhaps the church most similar liturgically and theologically in Washington to Dutch Calvinism.
The Shaping of the United Church of Christ: An Essay in the History of American Christianity, Louis H. Gunnemann; Charles Shelby Rooks, ed. Cleveland: United Church Press, 1999.
The Columbia Encyclopedia, Sixth Edition. 2001–2005.